In the continuous casting of steel to produce in a single operation shapes equivalent in section to conventional semifinished shapes, molten steel is poured from a tundish or ladle into the top of a cooled mold having a vertically throughgoing passage. The steel cools and at least its surface hardens in the mold, so that a cast strand can be pulled continuously by pinch rollers from the bottom of the mold. The mold is frequently reciprocated continuously vertically in order to prevent the casting from adhering to it, as such sticking could cause a breakout of molten steel in or below the mold. Between the lower end of the mold and the traction pinch rollers that pull the casting from the mold it is standard practice to spray water on the casting to harden it. This procedure, which is described in some detail in The Making, Shaping and Treating of Steel (H. E. McGannon; US Steel: 1971 p. 706ff), is extremely difficult to make work successfully for a long run, but represents such important economies in the production of steel that it is a highly developed art.
One of the parameters which is absolutely critical is the melt level, or the vertical level of the upper surface of the liquid steel in the mold. This level must be maintained above a lower limit so that the casting is sufficiently strong as it exits from the mold not to break out, and below an upper limit which would cause the mold to overflow or the casting to be so rigid as it exits the mold that, for example, it could not be bent. The range between these two limits is quite small, and there are only two principal ways to affect the melt level: by withdrawing the casting at a greater speed to lower it or more slowly to lift it, or by reducing the fill rate to lower it or increasing the fill rate to lift it.
Any such level measurer must operate under extremely adverse conditions of heat, airborne particles, and corrosive gases and vapors. In addition the level-measuring device must not interfere with pouring or vertical reciprocation of the mold, and must be able to determine melt level accurately when it is underneath a powder or slag layer, and even when the entire system is enclosed and filled with an inert or special treatment gas. In fact there is not at present any reliable level measurer which is usable in the continuous casting of billets or blooms.
German Patent document No. 2,101,729 describes a system that makes measurements of the currents that are induced in a conductor by a moving magnetic field. The secondary field of these induced currents can be sensed to determine the presence or position of this conductor.
Another system is described in French Pat. No. 2,251,811 which measures the impedance of a coil in whose moving alternating-current field an conductor is moved. This coil is juxtaposed with the top of the melt, which obviously is of conductive material while whatever above it is not, and can therefore roughly detect the melt level. To this end two U-shaped inductor cores are employed which are parallel to each other but whose coils are oppositely connected. Such inductors must be aligned with an opening or nonmagnetic window in the mold at the melt level. It is unfortunately impossible to provide such a window in a steel-casting mold which is normally made of thick water-cooled copper in which eddy currents wold form that would create a powerful secondary field completely masking the secondary field created by the eddy currents formed in the melt inside it. Arranging the system to hang down inside the mold has not worked out, as the level meter gets in the way and is quickly destroyed by the heat and corrosive chemicals generated by the process.
Commonly owned earlier U.S. Pat. No. 4,279,149 describes a method which eliminates the influence of the mold. In this system an alternating-current field-forming primary coil and two oppositely connected similar secondary coils as well as the liquid metal form a system in which the position of the melt level relative to the coils produces an induced voltage. The primary and secondary coils engage without contact coaxially around the mold without touching it. As a result of the position of the melt level, the voltages induced in the secondary coils as well as the electrical conductivity of the melt are measured, so that the detected voltage must be corrected for the particular conductivity to determine the melt level. Such an arrangement can only be used for small tubular molds due to the necessity of providing annular coils surrounding the mold and the difficulty of providing such arrangements on a large scale.